1. Field of the Invention
The present invention relates to the field of telecommunication and in particular to that of wireless telecommunication. The present invention is more particularly related to the multiplexing of radio signals emitted by different terminals sharing the same frequency resource.
2. Description of the Related Art
The main known techniques for multiplexing radio signals in current telecommunication networks, to enable different terminals to access a shared frequency resource to communicate with a station, consist mainly in multiplexing said radio signals:                by emitting them in different frequency subbands (“Frequency Division Multiple Access” or FDMA);        by emitting them at different time intervals (“Time Division Multiple Access” or TDMA);        by spreading their frequency spectrum by means of codes that are generally substantially orthogonal to each other (“Code Division Multiple Access” or CDMA).        
Each of these techniques is based on mechanisms for assigning physical channels (frequency subbands, time intervals, spreading codes) to the different terminals which are often very complex, dynamic and centralized at the station or, more generally, at the network.
These assignment mechanisms require a strict time and/or frequency synchronization of a terminal with the station on the one hand and with the other terminals on the other hand, via an often complex and energy-consuming protocol, as this requires each terminal to be regularly switched on.
It is understood that these assignment mechanisms are incompatible with very low data rate telecommunication systems (a few bits per second), as these data rates are insufficient to maintain synchronization between the terminals and the station, and/or with very long-range telecommunication systems (several tens of kilometers in rural areas), as maintaining synchronization between distant terminals is very difficult (propagation delay, Doppler effect, etc.).
This invention aims at overcoming the aforementioned problems, and in particular at proposing a method for using a frequency resource shared between several terminals, that is both simple and inexpensive to implement, while guaranteeing a low level of collisions between radio signals emitted by different terminals.
One advantageous, although in no way limitative application of this invention involves low data rate information collection systems such as sensor networks wherein sensors repeatedly emit data representative of the physical parameter measured to a data collection station. One non-limitative example involves sensors embedded in electric or gas meters, emitting electricity or gas consumption data to a collection station in order to establish an invoice for this consumption.
According to a first embodiment, this invention relates to a terminal comprising means for transmitting data towards a station in the form of radio signals, said radio signals being transmitted using a frequency resource shared between a plurality of terminals. Said terminal is configured to emit radio signals, the instantaneous frequency spectrum of which has a bandwidth that is significantly lower than a frequency drift of a frequency synthesis means of said terminal.
According to specific embodiments, the terminal comprises one or several of the following characteristics, which can be considered alone or according to any technically possible combination.
Preferably, the terminal is configured in a static manner to transmit radio signals in a single predefined frequency subband of the frequency resource, or according to a single predefined sequence of frequency subbands of said frequency resource.
Preferably, the terminal is configured to determine, in an autonomous manner, when to emit radio signals in a frequency subband of the shared frequency resource.
Preferably, the terminal is configured to determine, in an autonomous manner, what power to use to emit radio signals in a frequency subband of the shared frequency resource.
Preferably, the radio signals are signals that have been previously spread by means of a spreading code. Advantageously, said terminal is configured in a static manner to use a single predefined spreading code or a predefined sequence of spreading codes.
According to a second embodiment, this invention relates to a telecommunication system comprising:                a plurality of terminals according to the invention,        at least one station comprising means for detecting and decoding radio signals emitted simultaneously in different frequency subbands of the shared frequency resource.        
Preferably, the telecommunication system station comprises a FFT block adapted to determine a frequency spectrum of the shared frequency resource and a detector block adapted to search for at least one energy peak in the frequency spectrum determined, likely to correspond to a radio signal emitted by a terminal.
According to a third embodiment, this invention relates to a method for using a frequency resource shared between a plurality of terminals to emit data in the form of radio signals to at least one station, wherein each terminal emits radio signals, the instantaneous frequency spectrum of which has a bandwidth that is significantly lower than a frequency drift of a frequency synthesis means of said terminal.
According to specific embodiments, the method for using a shared frequency resource comprises one or several of the following characteristics, which can be considered alone or according to any technically possible combination.
Preferably, each terminal is previously configured in a static manner to emit radio signals in a single predefined frequency subband of the frequency resource, or according to a single predefined sequence of frequency subbands of said frequency resource.
Preferably, each terminal determines, in an autonomous manner, when to emit radio signals in a frequency subband of the shared frequency resource.
Preferably, each terminal determines, in an autonomous manner, what power to use to emit radio signals in a frequency subband of the shared frequency resource.
According to a fourth embodiment, this invention relates to a method for manufacturing terminals designed to use a shared frequency resource. For the manufacture of each terminal, said method of manufacture comprises the following steps:                obtaining at least one random number or pseudo-random number according to at least one generator generating random or pseudo-random numbers distributed according to a substantially uniform law,        determining a frequency subband of the frequency resource or a sequence of frequency subbands of said frequency resource according to the at least one random or pseudo-random number,        equipping said terminal with a frequency synthesis means configured in a static manner to transmit radio signals only in the frequency subband, or only according to the predefined sequence of frequency subbands of said frequency resource determined according to the at least one random or pseudo-random number.        
According to specific embodiments, the method of manufacture comprises one or several of the following characteristics, which can be considered alone or according to any technically possible combination.
Preferably, each terminal is equipped with a frequency synthesis means, the frequency drift of which is significantly greater than a predefined bandwidth of the instantaneous frequency spectrum of the radio signals to be emitted by this terminal.
Preferably, the step for determining the frequency subband of the frequency resource or the sequence of frequency subbands of said frequency resource to be assigned in a static manner to a terminal comprises at least one of the following steps:                selecting a fractional division value for a fractional frequency synthesizer,        selecting a control voltage value to be applied to a voltage-controlled oscillator,        selecting a group of components to be installed within the terminal to modify the oscillation frequency of an oscillator,        selecting a physical patch to be made on at least one component to modify the oscillation frequency of an oscillator.        